USING HYDROLOGIC SYSTEMS ANALYSIS AND GIS IN DEVELOPING CONCEPTUAL SITE MODELS FOR SUSTAINABLE GROUND WATER RESOURCES EVALUATION OF THE SNOWMASS AND CAPITOL CREEK AREAS, PITKIN COUNTY, COLORADO

A GIS-based, step-wise ground water resources evaluation procedure was created for land use planning and management by resource managers in Pitkin County, Colorado. The procedure addresses three concerns: 1) ground water resources availability; 2) long-term water supply sustainability; and 3) the water supply vulnerability to contamination. Hydrologic Systems Analysis (HSA), in combination with a Geographical Information System (GIS), were used to develop conceptual site models for the Snowmass and Capitol Creek (SCC) study area. The study area has significantly changing land use from agriculture to urbanization related to ski resort development. Results show that the hydrogeologic framework of the SCC hydrological system has Quaternary glacial moraines, terrace gravels, and modern alluvium comprising the major source aquifers of interest, and the Dakota/Burro Canyon, Ft. Hays, and Mancos Sandstone bedrock aquifers serving as minor aquifers. These systems have hydrologic inputs and outputs based on infiltration of precipitation and snowmelt; gaining or losing streams; springs and seeps; vegetation evapotranspiration and wetlands; topography, geomorphology and soils; human activity (irrigation ditches and irrigation, urbanization, ISDS); and geology. Based on the hierarchical approach, no regional sustainable system has been identified, whereas local scale ground water flow systems dominate the SCC area. Five types of conceptual site models are identified: 1) Upper Snowmass Creek Subsystem; 2) Lower Snowmass Creek Subsystem; 3) Upper Capitol Creek Subsystem; 4) Lower Capitol Creek Subsystem; and 5) Ft. Hays/Dakota-Burro Canyon Bedrock Subsystem. The most important transient anthropogenic hydrologic system parameters are decreased ground water recharge from irrigation return flow and leaky irrigation ditches, and increased ground water discharge from wells. As water rights and allocations change for these ditches, the hydrodynamics of the shallow aquifers will change, and water supplies from ground water may decline or vanish. All five sites are vulnerable to ground water pollution due to the absence of protective low-permeability hydrogeologic units between the ground surface and the aquifer units.